Internal combustion engines may include water injection systems that inject water from a storage tank into a plurality of locations, including an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Injecting water into the engine intake air may increase fuel economy and engine performance, as well as decrease engine emissions. When water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to the water. This heat transfer leads to evaporation, which results in cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx, while a more efficient fuel mixture may reduce carbon monoxide and hydrocarbon emissions. As mentioned above, water may be stored in a vehicle to provide water for injection on demand. However, in order to meet the water injection demands of an engine, a vehicle needs to have a sufficient supply of water. In one example, a water storage tank of a water injection may be manually refilled by a vehicle operator. However, in some situations, water for refilling the tank, such as distilled water, may not be readily available and having to re-fill the tank may be undesirable for the operator.
Other approaches to refilling a water storage tank includes collecting water (or condensate) from other vehicle systems on-board the vehicle, such as collecting water from an air conditioning (AC) system. For example, the approach shown by Kohavi and Peretz in US 2011/0048039 includes extracting water from an air conditioning system. Therein, collecting condensate is based on an amount of water stored in a water storage reservoir (e.g. tank). However, the inventors have recognized potential issues with such methods. In particular, collecting water opportunistically from an AC system when the AC system is already operating may be insufficient to meet the water injection demands of an engine. Conversely, running the AC compressor using power supplied by an engine (e.g., a mechanical AC system) independently and/or in addition to operator demand based on a water level in the water storage reservoir may decrease the fuel economy benefit of water injection.
In one example, the issues described above may be addressed by a method for a vehicle including adjusting an AC compressor load of a mechanical AC system and an amount of friction brake effort to deliver a driver demanded braking effort during a braking event based on a level of water in a water reservoir coupled to a water injection system. A water injection system, including the water reservoir, may be fluidly coupled to the mechanical AC system. Thus, when the AC compressor is run (e.g., as the AC compressor load is increased), water may be collected from the mechanical AC system and stored at the water reservoir for use in the water injection system. In this way, the AC compressor may be operated during a braking event to collect water for a water injection system, thereby providing water for injection via the water injection system. For example, adjusting the AC compressor load and the amount of friction braking may include increasing the ratio of the AC compressor load to friction braking during a braking event to collect water from the AC system in response to the water level in the water reservoir (e.g. tank) being less than a threshold level. In this way, the AC compressor may be operated to collect water for the water injection system and a desired brake effort may be delivered. As a result, the water reservoir of the water injection system may be replenished automatically without manual filling. Further, by running the AC compressor to collect water for injection during a braking event, the AC compressor may be operated without added fuel injection at the engine (e.g., kinetic energy from the vehicle may be used to run the compressor). As a result, fuel economy may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.